Borehole logging methods and apparatus

ABSTRACT

A method utilised in borehole logging, such as in surveying or exploration relating to a subsurface formation. The method includes deploying a logging instrument that includes a pressure sensor into a borehole drilled into the formation. The method includes the steps of obtaining a first pressure value at a first depth in the borehole, obtaining at least one further pressure value subsequent to the first pressure value during withdrawing or advancing the logging instrument in the borehole, and determining one or more characteristics of the subsurface formation, utilising at least one of the further pressure values, or a change in pressure (Δρ) between the first pressure value and a said further pressure value or values, or a change in pressure (Δρ) between a said further pressure value and another said further pressure value, or a combination of two or more of such values.

FIELD OF THE INVENTION

The present invention relates to borehole logging methods and apparatus,such as for survey or exploration relating to subsurface formations.

CROSS REFERENCE

This application claims priority from Australian provisional patentapplication 2013904475 filed on 19 Nov. 2013, the contents of which areto be taken as incorporated herein by this reference.

BACKGROUND TO THE INVENTION

Borehole logging is the method of making measurements and recordinginformation about formations of the subsurface geology and/or the depthand angles of the borehole from the surface.

A log is recorded by visual inspection of rock samples from core samplesextracted from the borehole and/or geophysical information obtained byone or more instruments lowered into the hole after the hole is drilledi.e. in an open hole.

Typically, the instrument recording the log is lowered into the holeusing a winch. Logging is performed in boreholes drilled for mineral oroil and gas exploration or during groundwater geothermal, environmentalor geotechnical surveying.

‘Wire-line’ logging is done in an open hole to acquire a completepicture of the rock properties of the subsurface formations. Thisinformation helps drilling operators and geologists to make decisionsabout drilling direction, presence/absence and direction of the resourcebeing sought, or mining production.

A wire-line instrument is lowered down the hole to a chosen depth sothat the petro-physical properties of formations can be captured andstored in a memory device for later analysis.

One or more sensors associated with the instrument can be activated tomeasure electrical, electromagnetic, natural gamma radiation levels oracoustic information to build a series of overlayed logs.

Traditional logging tools are connected via a cable connection thatprovides power and also provides a conduit to capture data in real timeor raw data from the instrument is fed to a data acquisition system forlater analyses.

Open hole logs are usually run before a hole is lined or cased off andthey are managed by a costly technical team. Open hole survey costs areextremely high for the miner or explorer. Therefore, only a few holescan be logged, limiting the information available to a geologist whenmaking drilling decisions.

‘Logging while drilling’ (LWD) technique has been pioneered in the oiland gas industry. LWD and also ‘measure while drilling’ (MWD) logstypically use mud pulse technology to transmit data from the downholetool/instrument in the borehole for onward transmission to the surfacefor continuous analysis.

This technique provides the same information as wire-line logging,however, instead of lowering the instrument into the borehole on awinched cable, the required sensors are embedded in the drill string andthe measurements are fed to the operator at the surface in real time toallow the drilling operators and geologist to obtain logs as well asinformation such as hole direction, weight on bit etc.

Data rates are slow (10 bits a second) and it is necessary to use datacompression techniques and buffering to the tool's on board memory. Slowdata transfer rates, complex technology to pass the data in any reliableway through mud pulses, and the need for a costly site technical teammakes the use of such mud pulse technology undesirable. It has beenrealised that a more efficient data gathering methodology is desirable.

Geophysical methods can be used to interrogate a larger expanse ofsubsurface formations and can be used in conjunction with geologicalinformation from core samples to build a better geological prediction ofthe formations surrounding the core holes.

A geophysical measuring instrument, commonly referred to as a logginginstrument or sonde, is lowered into the bore hole to collect relevantdata. A sonde is essentially a ‘probe’ in the form, for example, of anelectronic instrument arranged to sense one or more parameters orcharacteristics.

Logging instruments are already used in oil field operations to obtaininformation about the bore hole. This process is referred to as“logging”. Logging can be performed by “wireline logging”, “loggingwhile drilling” (LWD) and “through-the-bit logging” after drilling hasoccurred.

In wireline logging, a logging instrument is lowered into the bore holeafter the drill string has been extracted. The logging instrument hangsand is supported by a length of cable or “wireline”. Additionally, thewireline facilitates the electrical and communication connectionsbetween the logging instrument and the related equipment normallylocated at ground level.

In LWD the actual drilling assembly includes sensing instruments thatmeasure required parameters as the bore hole is being drilled. Suchsensing instruments are thus subject to the hostile downhole environmentand consequently their operation is often compromised.

Through-the-bit logging involves introducing a logging instrument, forexample a wireline tool, into the borehole through a central port in thedrill bit located at the downhole end of the drill string. The logginginstrument is lowered or pumped into the borehole through the interiorpassage of the drill string. The logging instrument is then passedthrough the port in the drill bit to enable logging of the boreholebelow the drill bit. Further, the instrument can be used to log thelength of the borehole as the drill string is pulled out of theborehole. This process is often referred to as “logging while tripping”.

U.S. Pat. No. 8,443,915 describes different through-the-bit loggingsystems used for logging well bores drilled to extract crude oil and/ornatural gas. As shown in FIGS. 1B and 1C of U.S. Pat. No. 8,443,915, abottom hole assembly (BHA) includes a mill bit, a mud motor, a loggingtool, a centralizer, a hanger and a disconnect. Once the well bore hasbeen drilled to the required depth, the mill bit is used to cut throughthe nose of the drill bit to establish the port for the logging tool topass through. The BHA is lowered through the bore of the drill stringuntil the hanger is seated against an adapter of the drill bit.

Although logging instruments are used in oil and gas drillingoperations, the use of such instruments has not readily translated tocore sampling operations. This is due to the costs associated withproviding specialist technicians and equipment at sampling sites tocollect and record geophysical data.

The discussion of the background to the invention herein is included toexplain the context of the invention. This is not to be taken as anadmission that any of the material referred to was published, known orpart of the common general knowledge as at the priority date of thisapplication.

One or more forms of the present invention seeks to provide at least amethod and/or logging instrument that enables geophysical datacollection as part of a core sampling process, preferably utilising atleast part of the drill rig equipment associated with core sampling.

One or more forms of the present invention has/have been developed withthe aforementioned problems in the known art in mind.

It has been found desirable to develop geophysical logging apparatusand/or one or more methods of use and/or deployment thereof, or todevelop a downhole logging method, that can be utilised or employed bydrill operators/personnel as part of their normal operations without theneed for additional specialist geophysical logging personnel.

Utilising the drill rig personnel/operators removes the need forspecialist personnel and avoids interrupting drilling operations oravoids delays in otherwise waiting for specialist personnel to arrive onsite after drilling. Logging operations can advantageously be carriedout by the drill operators/personnel who require less training orspecialisation.

Such an approach to drill logging operations is desirable in that ithelps reduce the significant costs associated with exploration ofsubsurface formations, and is intended to also lower the barrier toadoption of the technology in a sector that is historically conservativeto new technology and methods of working.

SUMMARY OF THE INVENTION

With the aforementioned in mind, an aspect of the present inventionprovides a method utilised in borehole logging, such as in surveying orexploration relating to a subsurface formation, the method including:

-   -   a) deploying a logging instrument into a borehole drilled into        the formation, the instrument including a pressure sensor;    -   b) obtaining a first pressure value at a first depth in the        borehole;    -   c) obtaining at least one further pressure value subsequent to        the first pressure value during withdrawing the logging        instrument from borehole;    -   d) in determining one or more characteristics of the subsurface        formation, utilising:        -   i. at least one of the further pressure values; or        -   ii. a change in pressure (Δp) between the first pressure            value and a said further pressure value or values; or        -   iii. a change in pressure (Δp) between a said further            pressure value and another said further pressure value; or        -   iv. a combination of two or more of i) to iii).

Preferably the first pressure value may be used as a reference valuerelative to which the further pressure value(s) is/are determined. Thus,change in pressure may be determined relative to the first pressurevalue as a reference value.

The first pressure value obtained by the logging instrument may be usedto determine actual depth of the logging instrument at that level withinthe borehole. For example, the first pressure value may correlate withknown data for expected pressure at known depths in a borehole.

Alternatively, if the actual depth of the logging instrument within theborehole is known or is, or can be, calculated, the first pressure valuemay be used as a cross reference with the known depth.

Change in pressure may be used to calculate or determine actual depth ofthe logging instrument in the borehole. A quantum of change in pressureor subsequent further pressure value may be compared with a previouspressure value (such as a previous further pressure value or the firstpressure value) and may be used to determine actual depth of the logginginstrument within the borehole.

It will be appreciated that depth within the borehole can be determinedas distance along the borehole or, alternatively, a vertical distancefrom the surface to a point in the borehole, such as the end of theborehole or to the instrument. Preferably, for one or more forms of thepresent invention the depth relates to distance into the borehole fromthe surface or from the start of the hole.

As the logging instrument is withdrawn from the borehole, the change indepth may be calculated or determined from change in pressure relativeto the reference first pressure value, which change can also be comparedwith removal of known length sections of the drill casing i.e. 3 metreor 6 metre lengths of casing.

A drop in pressure will be sensed each time a pressure reading is takenduring withdrawal of the logging instrument from the borehole. Thus,pressure values will decrease as the logging instrument is withdrawn,and the subsequent change in pressure (lower pressure Δp) will bedetected.

The logging instrument may include one or more sensors arranged andconfigured to project into the borehole beyond a drill bit at a distalend of the drill string.

The one or more sensors, such as one or more pressure sensors and/oraccelerometer(s), may be housed in a sonde. A sonde is an instrumenthousing one or more sensors and used downhole to gather data.

The drill bit may have an aperture through which the sonde sensor(s) maybe advanced. One or more pressure values may be sensed by the sensor(s)beyond the drill bit.

Pressure and change in pressure will be detected in fluid present in theborehole. The fluid may be a liquid, such as water or drilling mud, in a‘wet’ borehole or may be one or more gases, such as air, in a ‘dry’borehole.

One or more embodiments of the present invention may also includedetecting acceleration. For example, the sonde may include orcommunicate with at least one accelerometer. Preferably, the at leastone accelerometer is housed in the logging instrument or sonde.

Acceleration (or deceleration) may be used in conjunction with the firstand/or further pressure values in detecting depth in the borehole. Depthvalues can be correlated with gamma radiation readings from one or moregamma detectors to create a log of a gamma signature of the subsurfaceformation. The logged signature can be used to determine the structure,such as strata and/or type(s) of rock/deposits of the subsurfaceformation.

The logging instrument may have an onboard power supply, and may operateautonomously when downhole. Thus, the logging instrument may recordpressure and/or acceleration values during the withdrawal operationwithout requiring additional input from the operator at the surface.

The logging instrument may record pressure and/or acceleration, andother parameters as required, continuously or discontinuously. Forexample, the logging instrument may obtain or sample pressure values atpredetermined time intervals, such as every 2-5 seconds, or may usedetection of acceleration or lack of acceleration as a signal to obtaina next pressure value.

The pressure values may be obtained by the pressure sensor continuously,and the pressure values thus obtained may be sampled periodically.Electronics within the logging instrument may be used to sample thecontinuously obtained pressure values or to sample periodically fromrecorded pressure values within a processor or non-volatile memory ofthe electronics.

The logging instrument may cease sensing pressure for a period of timeuntil acceleration is at or above a particular threshold value. It willbe appreciated that acceleration can be a negative value i.e. adeceleration.

A gamma radiation detector may be provided within the downholeequipment, such as in the logging instrument, in the sonde and/or withinanother portion of downhole equipment, such as an electronic surveyinstrument, shuttle, core orientation device.

Alternatively, pressure values may be obtained for use with a downholeelectronic instrument that does not have or does not use a gammaradiation detector.

Pressure values obtained during deployment or recovery of the downholeinstrument may be correlated to other recorded data associated withcharacterising the subsurface formation. For example, the instrument mayinclude an electronic survey instrument, shuttle, core orientationdevice or other probe device.

Preferably, the gamma detector is provided within the sonde to projectinto the borehole when advanced beyond the drill bit. This means thatthe gamma detector is not shielded by surrounding metal as much as it isotherwise when within the drill string. Thus, the gamma detector can beprovided as a less sensitive, smaller and/or lower powered detector thanwould otherwise be needed if it remained surrounded by the metal wallswithin the drill string which otherwise severely attenuate the externalgamma signal.

The aperture through the bit restricts the size/choice of gamma detectorand limits the size of the detector's crystal element for detectinggamma radiation. Therefore, a larger or more numerous crystals may beemployed if the gamma detector remains within the core tube becausethere is more width/space within the tube compared with the narroweropening through the drill bit. Larger or more numerous gamma detectioncrystals help to detect a greater density of gamma radiation. This canhelp speed up gamma radiation recording. Suitable gamma radiationdetection crystals can include, for example, caesium iodide or sodiumiodide.

Values of detected (natural) gamma radiation can be associated with orcorrelated with respective pressure value or change in pressure withinthe borehole.

Natural gamma results from radioactive decay of naturally occurringradio-isotopes (e.g. potassium, thorium and uranium). Density ofsubsurface formations may also be detected or determined, such as bycorrelating measured gamma radiation values with absorbed gammaradiation by the rock from a source of gamma radiation carried by thelogging instrument.

Other signals may be used in combination with natural gamma signals todetermine density values or estimates for the surroundingrock/formation. For example, data or values from sound signals may beused in conjunction with gamma radiation values to determine density ofthe surrounding formation.

It will be appreciated that detected gamma radiation signals can be usedto provide an indication of the structure of the surrounding formationup to around 30 cm radius from the logging instrument or sonde or otherprobe carrying the gamma radiation detector.

Gamma rays emitted from the surrounding rock are absorbed by thedetector, resulting in a signal indicating the amount of gamma radiationover time. Different lithological formations have varying abundance ofthese radioisotopes, and the log of natural gamma can be used as alithological indicator. Referencing the measured gamma to depth in theborehole helps to give an indication of the subsurface formation (e.g.strata and formation/deposit types and position).

As the logging instrument is withdrawn up the borehole in stages(thereby producing distinct detectable pressure changes), gammaradiation can be detected and the values correlated with the respectivechange in pressure or the fact that a pressure change has occurred. Inthis way, gamma radiation and pressure can be correlated and associatedwith depth.

Gamma radiation gives an indication of the type of deposits orsub-surface formation the gamma detector has passed through. Changes indetected gamma radiation can be used to determine or predict the typesof rock or deposits present under the surface, and changes in strata orrock type as values vary.

The logging instrument or sonde can include an array of sensors tomeasure electrical, electromagnetic, natural gamma radiation levelsand/or acoustic information. A series of logs relating to these detectedvalues can be produced to give overlying data sets.

A gamma radiation detector can be provided as part of the logginginstrument. Preferably the gamma radiation detector remains within thedrill string when the pressure sensor is advanced beyond the drill bit.

Preferably, gamma radiation values may be obtained when the logginginstrument has ceased movement for a period of time during thewithdrawal or advancement in the borehole.

Alternatively, gamma radiation values may be obtained when the logginginstrument is moving over a period of time during the withdrawal oradvancement in the borehole. Preferably the rate of movement is known orestimated, such as between 1 m/min and 20 m/min, and preferably between2 m/s and 12 m/min, and more preferably around 10 m/min. However, sloweror faster movement can still be used to record useable gamma radiationvalues.

The obtained gamma radiation values may be correlated with pressurevalues to provide a gamma radiation value associated with depthpositions within the borehole.

The logged gamma radiation values may be correlated with logged depthvalues obtained from the corresponding pressure or change in pressurevalues to create a lithological plan of the structure of the subsurfaceformation.

BRIEF DESCRIPTION OF THE DRAWINGS

At least one embodiment of the present invention will hereinafter bedescribed with reference to the accompanying drawings, in which:

FIG. 1 shows a logging instrument deployed within a core barrel with asonde projecting beyond the instrument's drill bit, according to anembodiment of the present invention.

FIG. 2 shows a diagrammatic representation of the general arrangement ofcomponents within the logging instrument according to an embodiment ofthe present invention.

FIG. 3 shows a diagrammatic representation of the logging instrumentdeployed in a borehole with the sonde projecting beyond the drill bit,according to an embodiment of the present invention.

FIG. 4 shows a chart of pressure versus time for a logging instrumentadvanced into and subsequently withdrawn in stages from the borehole,according to an embodiment of the present invention.

FIG. 5 shows a chart representing gamma radiation readings with respectto acceleration values according to an embodiment of the presentinvention.

DESCRIPTION OF PREFERRED EMBODIMENT

One or more forms of the present invention can be employed in thefollowing practical logging applications/apparatus:

-   -   Wire line open hole logging: with depth encoder plus pressure        sensing plus accelerometer depth correlated.    -   Wire line thru the bit logging: with depth encoder plus pressure        sensing plus accelerometer depth correlated.    -   Core barrel coupled through the bit: similar to Wire line open        hole logging and Wire line thru the bit logging above, but also        coupled directly to the core barrel: with pressure sensing plus        accelerometer depth correlated.    -   Instrumented core barrel and/or shuttle.

It will be appreciated that embodiments of the present invention are notlimited to obtaining pressure values to correlate solely with gammaradiation values. The obtained pressure values may be used inconjunction with other data gathered. For example, a downhole surveyinstrument, core orientation tool or shuttle may be deployed andpressure values obtained during deployment or recovery from downhole.Alternatively or in addition, other data may be obtained, such asmagnetic, gravity and temperature data from down the hole can be used,correlated or combined with the obtained pressure data to aid indetermining one or more characteristics of the subsurface formation.

At least one embodiment of the present invention will hereinafter bedescribed in relation to a practical application of the embodimentfollowing a coring operation (e.g. core sampling).

During normal coring operations, the drill bit cuts through the rock andthe core is forced up into the core tube. The drill string is thenpulled back at the surface and the rock is snapped from the earth. Thecore is taken to the surface for analysis.

After the core is removed from the tube at the surface, a logginginstrument (geo-physical measuring device) is attached to the empty coretube for deployment into the hole to detect characteristics of the rockproperties around the borehole.

The logging instrument is connected to the core tube using a mechanicalcoupler. The logging instrument is then advanced with the core tube intothe borehole to record the geophysical data, e.g. gamma radiationemitted from the rock surrounding the borehole and/or borehole angle(orientation).

The core tube is usually pumped into place or dropped by a special wireline mechanism.

Advantageously, the core tube that was used to retrieve the coresample(s) is also used to deploy the logging instrument into a positionwhere it can sense the properties of the subsurface formation in theempty hole. Thus, because the logging instrument is self contained i.e.self powered and operates autonomously once deployed, and the originalcore tube is re-deployed, the drill operators can deploy and retrievethe logging instrument without the need for specialist personnel ortechnical training. This provides significant operational cost savingsby avoiding delays waiting for the specialist team to arrive. Loggingcan commence almost immediately after the core sample has beenrecovered.

The drill string (and therefore the logging instrument) is withdrawnfrom the borehole and each drill rod is removed at the surface. Thepressure sensor within the sonde of the logging instrument sensespressure as it is withdrawn from a depth within the borehole. Changes inpressure values are used to determine change in depth.

Preferably, the logging instrument is connected to the core tube via anadapter, preferably made of stainless steel to resist corrosion andprovide strength.

When downhole adjacent the drill bit, the sonde with sensors associatedwith the logging instrument are advanced through an aperture in thedrill bit to extend into the borehole beyond the drill bit. Thus, thesensors are beyond the core barrel and drill bit.

Electronics, processing, memory and battery power components cantherefore remain within the body of the logging instrument inside thecore barrel, thereby protecting those logging instrument components.

Sensing, such as gamma and magnetic sensing by the sensors, is notdetrimentally influenced, or at least less affected, by the steel corebarrel and drill bit when the sensors project beyond the drill bitcompared with the entire logging instrument remaining within the steelcore barrel. However, it will be appreciated that the present inventionis not limited to having the sensors project beyond the drill bit.

The logging instrument utilised in an embodiment of the presentinvention can do one or more of the following functions:

-   -   automatically mode change, shut down, wake up, correlate data to        depth measuring pressure and preferably also detect movement        (such as acceleration/deceleration).    -   store the values to non-volatile memory.

Algorithms can be employed to discard data that is not required, therebysaving vital memory space. The logging instrument can also measuresurvey orientation and calculate the latitude and longitude position ofthe data and borehole.

Embodiments of the present invention provide the drilling operator withextra data (e.g. gamma rock properties information), whilst onlyperforming the one operation (that being a traditional magnetic/gravitydead reckoning survey). This saves time (and hence money) because onlyone pass is required to acquire all of the data.

One or more benefits of embodiments of the present invention arerealised in that:

-   -   logging and survey can be performed without the risk of the        borehole collapsing i.e. the drill string is still in place,    -   the drill operator(s) can perform the logging procedure without        any additional equipment or specialist technical training or        personnel,    -   the logging instrument is self-powered and has autonomous        operation,    -   no power or signalling needs to be transmitted ‘down the        wire’—no risk of broken power or communication wires i.e. the        logging instrument can be deployed on a simple steel wire,    -   non-volatile memory storage and also data transfer can be        wireless to a fit for purpose handheld device that will also        control the instrument operating and test modes at the surface        before deployment and after retrieval of the logging instrument,    -   automatic depth acquisition and correlation,    -   automatic wire line depth counter interface,    -   automatic calculation of depth at any time due to input of        number of rods, barrel length and stickup,    -   acceleration analysis to determine when a rod starts to be        pulled and subsequently stops. Fine depth ‘correlation’—Depth        interpolation can be calculated via time interval between rod        removals (rod removals involve start-stop removal of the drill        string as each section of rod is unscrewed from the next one and        then the drill string withdrawn another length of rod for that        rod to then be unscrewed, and so on),    -   automatic survey operation—during the rod removal/detachment        process there is a period of ‘rest’. This can be used as a        prompt to automatically initiate a magnetic/gravity survey to        take place.

The logging instrument 10 depicted in FIGS. 1 and 2 includes a core tubeadaptor or a landing collar 12, an optional spacer bar 14 when a surveyis required, and a sonde 16 housing sensors. The spacer bar 14 may notbe required when a gamma detector is advanced through the drill bitwithout a survey being required. Although only a single spacer bar 14 isdepicted, multiple spacer bars 14 may be included. Preferably, threespacer bars 14 are employed.

The logging instrument 10 is an autonomous tool and as such does notrequire external data or power cable connections.

The logging instrument 10 includes at least one gamma radiation detector20, which preferably remains disposed within the core barrel when thesonde portion is deployed beyond the drill bit. The gamma radiationdetector 20 can be deployed anywhere in the instrument housing. Thegamma radiation detector 20 may be deployed to stay within the tube toobtain higher density readings because the gamma radiation detector 20can be larger because it does not need to project through the smalleraperture though the drill bit.

The logging instrument 20 has on-board power from batteries 22.

During normal operation, a coring rig (without the logging instrument 10attached) is used to extract a core sample from a geological formation.Once the core sample has been extracted, logging of the core hole canoccur.

The logging instrument 10 is deployed into the borehole 180. Thisinvolves a series of steps readily accomplished by the drilloperator/personnel, and thus no additional specialist personnel orhighly trained logging technicians are required on site.

A method of deploying the logging instrument 10 includes the steps ofassembling the logging instrument 10; connecting the logging instrument10 to the core tube of the core rig; deploying the connected core tubeand logging instrument 10 through the core barrel 140 of the core riginto the borehole 180.

A further step can include seating the connected core tube and logginginstrument 10 so that a lower part of the logging instrument 10 islocated below the drill bit 160 on the core barrel 140.

The step of assembling the logging instrument 10 includes connecting theadaptor 12 to the core tube, connecting the spacer bar or bars 14 to theadaptor 12 and connecting the sonde 16 to the spacer bar 14.

The connected core tube 180 and logging instrument 10 is deployedthrough the drill string and into the core barrel 140 using a backendassembly.

FIG. 3 shows the logging instrument 10 deployed in a section of corebarrel 140 within a borehole 180.

In order to log a part of the length or the entire length of the corehole it is necessary to move the logging instrument 10 either into orout of the borehole. It will be appreciated that a preferred form ofcarrying out a method of the present invention is to deploy the logginginstrument 10 into the borehole and gradually withdraw it whilst takinggamma radiation readings (and any other sensor readings) periodicallyduring the withdrawal and as pressure drops due to the decreaseddistance into the borehole.

However, it will be appreciated that the reverse may be carried outwithin the scope of the present invention. That is, inserting thelogging instrument 10 into the borehole and taking gamma radiationreadings with the logging instrument 10 periodically or continuously asthe logging instrument 10 is advanced into the borehole and therefore aspressure is increasing with distance into the borehole.

FIG. 4 shows a graph of pressure against time. The logging instrument 10is inserted into the borehole from the surface entrance at or around 400seconds. Pressure gradually increases with depth as the logginginstrument 10 advances into the borehole, up to a maximum recordedpressure (and therefore a maximum depth for this example) correlating toaround 24 bar (correlating approximately to 240 m). The pressure valuesgiven on the ‘y’ axis are raw data values which require calibration tospecific pressure units. Typically, working pressure values are 0-5000psi, with up to around 8000 psi as a workable maximum.

It will be appreciated that pressure within a vertical borehole for agiven distance will vary compared with pressure in a borehole thatdeviates from vertical for the same distance within the borehole. Also,pressure will typically be higher for a given distance or depth in aborehole if the fluid surrounding the pressure sensor is denser thanwater, for example, drilling mud has a greater density than water orair. Hence, the pressure sensor would be calibrated before deploymentinto the borehole.

The logging instrument 10 is gradually withdrawn from the borehole, withdecreasing pressure being detected by the pressure sensor(s) housed inthe logging instrument 10. Pressure is periodically static for a fewseconds as the gamma radiation sensor(s) take(s) readings while thelogging instrument 10 is static. The downward slope of the graph showsthis as short periodic steps in the downward pressure slope to themaximum time of approximately 3150 seconds, after which pressure becomesstable, e.g. at the surface.

One or more accelerometers within the logging instrument 10 can be usedto detect changes in movement of the logging instrument 10 and thereforecorrelate that movement (or lack thereof) with respective gammaradiation readings gathered.

As shown in FIG. 5, the time period used is 1 second to 3597 seconds(approximately a one hour period). This is an example only, and otherperiods are envisaged within the scope of the present invention.

The pressure sensor can take continuous pressure readings 9 i.e. such aswhen the pressure sensor is an analogue device). Those pressure readingscan be sampled by electronics within the logging instrument. Samplingmay be periodic, such as every few seconds or fractions of a second.Millisecond sampling can be carried out. Preferably sampling is at arate of a sample every 0.5 seconds or less. Sampling at rates above 0.5seconds is also envisaged.

Obtaining gamma radiation values can be at a sampled rate of every fewseconds, more preferably around every second.

The initial portion of the graph ‘A’ shows pressure rising/ramping up asthe logging instrument 10 is deployed into the borehole. Pressureincrease is proportional to depth into the borehole.

The ramping down (section ‘B’) in FIG. 4 is the pulling out of the holeof the logging instrument 10 to a shallower depth (less pressure) and soon. The slope of the graph shows the rate of change of pressure withtime.

The graphs in FIGS. 4 and 5 depict simulated rod pulls from an estimateddepth of around 240 m, so the instrument 10 was pulled up and stoppedfor a couple of minutes, then pulled up and stopped again, and so on.

FIG. 5 shows detected natural gamma radiation from the surrounding rockformation as detected by the on-board gamma radiation detector 20plotted against estimated depth derived from the pressure valuesobtained during retrieval of the logging instrument 10 back up theborehole. The pressure and depth data, and gamma signals can all bestored in an on-board non-volatile memory within the logging instrument10 for recovery from the logging instrument 10 at the surface.

The simulated pulls are roughly 10 m/minute, equating to a typicalindustry logging speed of a gamma measurement.

When the borehole is a dry hole (i.e. an air filled hole rather than awet, water/drilling mud filled hole), a barometric pressure sensor canbe employed to detect air pressure and changes in air pressure downhole.

1. A method utilised in borehole logging, such as in surveying orexploration relating to a subsurface formation, the method including: a)deploying a logging instrument into a borehole drilled into theformation, the instrument including a pressure sensor; b) obtaining afirst pressure value at a first depth in the borehole; c) obtaining atleast one further pressure value subsequent to the first pressure valueduring withdrawing or advancing the logging instrument in the borehole;d) determining one or more characteristics of the subsurface formation,utilising: i) at least one of the further pressure values; or ii) achange in pressure (Δρ) between the first pressure value and a saidfurther pressure value or values; or iii) a change in pressure (Δρ)between a said further pressure value and another said further pressurevalue; or iv) a combination of two or more of i) to iii).
 2. The methodof claim 1, wherein the first pressure value provides a reference valuerelative to which the further pressure value(s) is/are determined. 3.The method of claim 1, wherein the first pressure value obtained by thelogging instrument is used to determine actual depth of the logginginstrument at that level within the borehole.
 4. The method of claim 3,wherein the first pressure value correlates with known data for expectedpressure at known depth in a borehole or the actual depth of the logginginstrument within the borehole is known or is calculated, the firstpressure value is used as a cross reference with the known or calculateddepth.
 5. The method of claim 1, wherein change in pressure (Δρ) is usedto determine actual depth of the logging instrument in the borehole. 6.The method of claim 1, wherein a quantum of change in pressure orsubsequent further pressure value is compared with a previous pressurevalue and used to determine actual depth of the logging instrumentwithin the borehole.
 7. The method of claim 1, including providing theone or more sensors housed in a sonde, and advancing the sonde beyond adrill bit at a distal end of the drill string within the borehole. 8.The method of claim 7, including advancing the sonde through an aperturethrough the drill bit.
 9. The method of claim 1, any one of thepreceding claims, wherein the one or more pressure values is/are sensedby the sensor(s) beyond the drill bit.
 10. The method of claim 1,wherein pressure and change in pressure (Δρ) are detected in a fluidpresent in the borehole, the fluid being a liquid in a ‘wet’ borehole orgases including air in a ‘dry’ borehole.
 11. The method of claim 1,including detecting acceleration of the logging instrument.
 12. Themethod of claim 11, including using acceleration data and the firstand/or further pressure values to determine depth of the logginginstrument to correlate with detected gamma radiation values.
 13. Themethod of claim 1, wherein the logging instrument operates autonomouslywhen downhole to record pressure and/or acceleration values during thewithdrawal or advancing operation without requiring additional controlinput from the operator at the surface other than controlling withdrawalor advancing of the logging instrument.
 14. The method of claim 1,wherein the logging instrument records pressure and/or acceleration, andother parameters as required, continuously, discontinuously orperiodically.
 15. The method of claim 14, wherein the logging instrumentsamples pressure values at predetermined time intervals.
 16. The methodof claim 15, wherein the predetermined time intervals are time intervalsbeing 2 seconds or less.
 17. The method of claim 15, including usingdetection of acceleration or lack of acceleration as a signal to obtaina said next pressure value.
 18. The method of claim 1, including ceasingsensing pressure for a period of time until acceleration is at or abovea threshold value.
 19. The method of claim 1, including obtaining orstoring detected gamma radiation values downhole using a gamma radiationdetector associated with the logging instrument.
 20. The method of claim19, wherein the gamma radiation detector is part of the logginginstrument.
 21. The method of claim 19, whereby the gamma radiationvalues are obtained when the logging instrument has ceased movement fora period of time during the withdrawal or advancement in the borehole.22. The method of claim 19, whereby the obtained gamma radiation valuesare correlated with pressure values to provide a gamma radiation valueassociated with depth positions within the borehole.
 23. The method ofclaim 22, including correlating the logged gamma radiation values withdepth values obtained from the corresponding pressure or change inpressure values to create a lithological plan of the structure of thesubsurface formation.